Wireless communication network systems usually comprise a plurality of Base Stations, defining one or more wireless coverage areas usually called cells or sectors. When a mobile User Equipment (UE) currently in wireless communication with a given base station starts to move out of the range of good wireless communication during the course of a call, the wireless network will attempt to find another base station, or another sector of the given base station, with which the mobile UE can continue wireless communication with minimal interruption. This process of transferring a mobile UE from one cell to another, or from one sector to another, is termed “handover” (HO)
Current standards for cellular mobile communications allow for preparing more than one target cell to receive a moving UE during handover. This is e.g. the case of inter Radio Access Technology (RAT) in Third Generation Partnership Project (3GPP) Long Term Evolution (LTE).
In LTE, even when more than one cell may be prepared for handover, via handover request messages, only one target cell is signalled to the UE, i.e. one dedicated preamble is sent in the handover command. The cell thus signalled will hereinafter be referred to as Primary Target Cell (PTC).
The other target cells, which have been prepared but not signalled to the UE, will hereinafter be referred to as Secondary Target Cells (STCs). Only in case the handover to the PTC fails the UE may try to access one of the STCs.
On handover failure, the UE attempts to re-connect to the best cell using the Radio Link Failure (RLF) recovery mechanism. This connection resumption succeeds only if the accessed cell is prepared, i.e. concerns a cell of the source network node or of another network node towards which handover preparation has been performed.
The best cell, receiving cell after handover failure, identifies the UE via the Cell Radio Network Temporary Identifier (C-RTI). The source cell (Cs) is identified via the Physical Cell Identity (PCI), which is not unique in the network, or the Global Cell Identity (GCI), which is unique in the network, if that is available at the UE.
If the best cell after handover failure is prepared for handover, it will inform the source cell of the arrival of the UE via a release resources message. It is assumed here that the source cell is also informed after a reconnection attempt to a non prepared cell, either by a specific message or implicitly by a UE context request.
However, there are some problems with the current solutions. Firstly, the set of STCs that are prepared for handover is a sub-set of a Neighbour Cell List (NCL), which lists a number of other wireless coverage areas to which handovers are preferably made. This sub-set of the NCL is hereafter called Secondary Target Cell List (STCL). Each prepared cell may need to allocate a dedicated preamble and reserve resources for a period of time. Secondly, a large STCL leads to an unnecessary load on the X2 interface interconnecting the cells, and reserved resources.
A way to reduce the cost of preparing a cell, would be having two different handover request messages for PTCs and STCs, but there will always be a cost associated with the procedure, e.g. additional signalling, and current standards do not allow for such a differentiation in the handover request message.
Document US 2003/0190916 A1 presents a method and system for optimizing a cell neighbor list in a CDMA system, wherein potential neighbor cells are removed from the cell neighbor list under certain conditions.
Document WO 2007/027033 A1 presents a method and apparatus for optimizing neighbor list automatically in synchronous CDMA network.